Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low energy X-ray from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed. In the reconstruction, the map of the linear attenuation coefficient (LAC) of the imaged subjects is obtained from the line integrals of the LAC through an inverse Radon transform. The line integrals can be related to the logarithm of the primary intensity of the X-rays passing through the subject. However, the measured X-ray intensity on the detector may include both scattering photons and primary photons. Thus, the images reconstructed from scattering, contaminated intensities may contain some scattering artifacts.
A third generation CT system can include sparsely distributed fourth generation, photon-counting detectors. In such a combined system, the fourth generation detectors collect primary beams through a range of detector fan angles, preventing the use of anti-scatter grids. Thus, for such a combined third/fourth generation system, scattering is a significant problem. In particular, for multi-slice CT, especially wide-cone CT, the scattered X-ray intensity can be equal to or higher than the intensity of the primary beam. Further, for spectral CT, the scattered X-ray intensity affects not only the detected count rate, but also the spectral measurement.
Conventionally, several schemes have been proposed for reducing scattering in third generation CT. However, none of these schemes is appropriate for sparsely distributed, fourth generation CT detectors.
For example, one conventional approach for third generation systems is the use of post-patient collimators to physically block scattering from reaching the detectors. However, this approach does not work for stationary, sparsely distributed fourth generation detectors, which need to receive X-ray beams for a large range of fan angles.
Further, conventional single-slice, fourth generation CT platforms have used a second row of detectors placed next to a first detector row for imaging in which signals from the second row are used to evaluate the scattering for the first detector row. However, this method does not work well when multiple rows of detectors are used for multi-slice CT.